Method and installation for producing metal strips and sheets

ABSTRACT

The invention relates to a method for producing metal strips and sheets involving the following method steps: pouring a strand, which is provided in the form ( 1 ) of thin slabs and which is comprised of carbon steel or special steel, into a casting machine ( 4 ) and rolling the cast product away from the casting heat. The aim of the invention is to utilize free capacities of existing installations, that is, installations not operating at full capacity, with which primary carbon steels are produced, in order to supplement other types of steel or to supplement cast products produced in another manner. To this end, the invention provides that, together with these first cast products ( 1 ), at least second cast products ( 2 ), which are produced in a second process route (II) from special steel if the first thin slabs are cast from carbon steel or are produced from carbon steel if the first thin slabs are cast from special steel, are rolled within a joint rolling program.

[0001] The invention concerns a process for producing steel strips and sheets, which comprises the following steps: casting a strand of carbon steel or special steel in thin slab format in a casting machine, passing the cast products through a reheating furnace to produce a uniform temperature, rolling the cast products in a rolling mill, cooling the rolled product on a runout table, and coiling the rolled product on a coiling machine. In addition, the invention concerns an installation for carrying out the process.

[0002] Thin slabs are defined here as cast products with a thickness of 30-130 mm, and especially 40-60 mm. EP 0 808 672 A1 describes a process and installation for producing special steel or carbon steel plate from thin slabs produced by continuous casting. In this process, a strand of thin slab 40-100-mm thick is cast either from molten special steel or from molten carbon steel and divided into individual thin slabs, which are conveyed through a heating line and then continuously rolled.

[0003] Stahl und Eisen, Vol. 115, No. 9, pp. 89-99, 1995 describes results for different grades of steel processed by the generic process, which is also known as “CSP” technology. These results include the direct charging of special steels, such as the material group of stainless Cr steel with C≦0.10% and Cr≧13% and the material group of stainless CrNi steel with C≦0.10% and Cr≧17%.

[0004] The goal of the invention is further development of a generic process and a generic installation for the purpose of making it more economical to produce strips and sheets made of special steel.

[0005] With respect to the process, this goal is achieved by rolling at least two series of cast products within a common rolling program, such that, if the first cast products produced in a first process route are cast from carbon steel, then the second cast products produced in a second process route are cast from special steel and vice versa. Thus, the crux of the invention is to integrate products of both types of steel in a common rolling mill in a generic installation and to roll both types of products within a common rolling program. The term “rolling program” is generally defined as the specified sequence of slabs to be rolled in the period of time between two roll changes.

[0006] The invention achieves optimal utilization of an installation, which would not be optimally utilized if used solely for the production of special steel sheets, which are produced in smaller amounts compared to carbon steels. In addition, greater variability of an installation in which carbon steels are produced is achieved by additionally rolling special steels. In principle, the invention is not limited to a rolling program for cast products made of the two types of steel; the rolling of additional types of steel in a rolling program is also conceivable, but carbon steels and special steels are always rolled together. The proposal of the invention to carry out the rolling in a common rolling program makes it possible to process two very different types of steel independently of the throughput capacity of an installation and of the steel production output.

[0007] The term “special steel” is basically defined in EN 10 020. In this regard, a distinction is made between unalloyed and alloyed special steels. The special steels in accordance with the invention include, for example, antifriction bearing steels, tool steels, creep-resistant steels, and high-grade structural steels. In particular, this term includes high-alloy steels with alloying elements and amounts that stabilize the austenitic structure towards lower temperatures, especially high-alloy austenitic CrNi steels.

[0008] To avoid loss of quality in the finished product, the rolling mill is adapted to the given material to be processed during the rolling program. Preferably, this means that the rolling mill is already being prepared for the next material to be processed in the rolling program, while the previous material is still being processed. In particular, this means that the negative effects of the rolling of the special steel on the surfaces of the work rolls, which usually take the form of rough surfaces and have an interfering effect on the subsequent rolling of the carbon steels, are eliminated during the operation or online. The negative effects of the carbon steel rolling in the form of a larger amount of loose scale, which would be pressed into the surface of the following special steel product, are similarly removed during the operation or online during the rolling of the carbon steels. The rolling mill is optimally prepared for the next rolling product in this way.

[0009] Alternatively or supplementarily, the rolling mill is also prepared for the following strip during the interruption in rolling between the rolling operations on two strips.

[0010] In a preferred embodiment, the first cast products in thin slab format or groups of them and the second cast products or groups of second cast products are alternately rolled. The term “alternation” can mean irregular or regular alternation of slabs of the two types of steel. A regular alternation, as is proposed as preferable in claim 5, and thus a regular succession of the relatively softer carbon steels and relatively harder special steels, produces a lower mean load for the drives of the work rolls, where a high load is immediately compensated by a lower load.

[0011] The second cast products, which are rolled in one rolling program together with the thin slabs of the first process route, may themselves be thin slabs or may be so-called conventionally cast slabs, which are primary rolled and then run into the common rolling mill. In the case of thin slabs, which are cast and cut in a second process route parallel to the first process route, the thin slabs are preferably passed, if necessary, through a second reheating furnace and a ferry, into a common reheating furnace and then the rolling mill. In the case of conventionally produced slabs, i.e., cast products with casting thicknesses up to 250 mm, primary rolling is carried out on a conventional hot-rolled wide strip mill in a reversing stand. The now cooled slabs, after being conveyed to an installation in which the first process route is being run, are reheated from the cold state in a slab furnace and assigned by a ferry to the reheating furnace of the first process route or to the rolling mill in the order preset in the rolling program. In principle, it is also conceivable for a primary rolled slab to be conveyed immediately without cooling for integration in the common rolling program. In this connection, it makes sense to homogenize the temperature before assignment.

[0012] The invention is not limited to the above-described possibilities of integration of thin slabs or conventionally continuously cast, primary rolled slabs; the integration of any type of cast product, e.g., products produced by strip casting, is conceivable, although preliminary products produced in this way already have small initial thicknesses.

[0013] With respect to the equipment, it is proposed that a generic installation be equipped with an integration unit for the introduction of at least second cast products made of special steel, if the first cast products from the first process line are cast from carbon steel, or for the introduction of at least second cast products made of carbon steel, if the cast products from the first process line are cast from special steel, and with a rolling mill for a combined rolling program for cast products made of carbon steel and special steel and preferably with means for preparing the rolling mill for the next rolling product made of the other type of steel.

[0014] In accordance with a preferred embodiment with respect to the means for preparing the rolling mill, grinding and/or polishing equipment is provided for the work rolls of at least one rolling stand, which counteract a rough roll surface during the rolling of the special steel product and/or during the interruption in rolling. This equipment, e.g., grinding equipment that presses against a work roll, is installed on the run-in side, preferably on all rolling stands of the rolling mill. At the same time, the rolling mill has rinsing equipment, which operates especially or to a greater extent during the rolling of the carbon steel products and/or during the subsequent interruption of rolling and frees the rolling mill of the loose scale from the carbon steels. Claims 15 and 16 propose additional equipment, which is provided alone or additionally in the rolling mill for the purpose of obtaining good surface qualities despite the rolling of second types of steel with different rolling properties and negative effects on the rolling mill and thus on the surface properties of the following products. Alternatively or additionally, rolls that are especially resistant to wear may be used. In particular, it is proposed that rolls be used which were produced by the powder-metallurgical HIP process as well as rolls made of high-speed steels.

[0015] Further details and advantages of the invention are found in the secondary claims and in the following description, which explains in greater detail the embodiments of the invention shown in the drawings. In this regard, in addition to the combinations of features specified above, features alone or in other combinations also fall within the scope of the invention.

[0016]FIG. 1 is a schematic representation of an installation for casting and rolling thin slabs with two casting strands and a common rolling mill in accordance with a first embodiment of the invention.

[0017]FIG. 2 shows the nature and sequence of a rolling program in diagram form.

[0018]FIG. 3 is a schematic representation of an installation for casting and rolling thin slabs with one casting strand for thin slabs and one process route for a conventionally cast product in accordance with a second embodiment of the invention.

[0019]FIG. 4 is a schematic representation of an installation for casting and rolling thin slabs with two casting strands and a common rolling mill in accordance with a third embodiment of the invention.

[0020]FIG. 1 shows an installation with two casting strands or two cast products 1, 2, in which the first process route I comprises a rolling mill 3 preceded by a casting machine 4. The first casting machine 4 has a tundish 5 and a funnel-shaped thin slab mold (shown schematically enlarged here) for casting thin slabs with thicknesses of 40-100 mm. The casting machine 4 itself is fed in the present example with molten special steel from ladle 7. The molten steel is produced, for example, by a blast furnace-converter route (labeled 8) or with an electric steel furnace followed by secondary metallurgical operations and then conveyed to the casting machine in the ladles 7 (the conveyance step is labeled 9).

[0021] The cast product in the form of the casting strand is turned from the vertical to the horizontal by rolls 10 a,b, and, in the present embodiment, is cut to length by a first set of shears 11. The individual thin slabs then enter a first reheating furnace 12, for example, a roller hearth furnace or rocker bar furnace, in which a uniform thin slab temperature is produced. The thin slabs then pass through a descaling system 13 and into the rolling mill, in which the desired final dimensions are produced. The finish rolling step 14 of the rolling mill consists here of six rolling stands (e.g., 15 a), each with two work rolls (16 a,b) and two support rolls (17 a,b). This is followed by a runout table 18 with cooling equipment 19 and a coiler 20 for winding the strip into a coil.

[0022] In a second, parallel process route II, the installation shown in FIG. 1 has a second cast product in the form of casting strand 2, which is made of carbon steel. This process route consists solely of the reheating furnace 21 with the same structural units as process route I described above, so there is no need to discuss it in further detail. The first process route I includes an integration unit for the thin slabs from the second process route II. This integration unit consists of a ferry 22, which can be transversely inserted into a section 22 a of the first reheating furnace 12 by moving the section 22 a to the other side.

[0023] In accordance with the invention, a common rolling program is operated in the rolling mill 3. An example of a rolling program of this type is illustrated in FIG. 2, in which the particular type of steel is plotted against the number of strips. In this example, the rolling program begins after the change of the work rolls with the rolls of a special steel cast product, followed by a cast product made of carbon steel. The sixth to eighth slabs are then made of carbon steel, and the twelfth to sixteenth as a group are made of special steel. FIG. 2 serves only as an example of a possible common rolling program.

[0024] The rolling of the special steel results in rougher roll surfaces, which would have disturbing effects on the immediately following rolling of carbon steel, such as, for example, the rolling in of scale and rougher strip surfaces. Therefore, the rolling mill has grinding and/or polishing machines installed on each of the rolling stands. For example, a machine of this type, which is labeled with reference number 23, acts on the upper and lower work rolls. In addition, an edge heater 24 is provided, which is installed in front of the descaling system 13. This heater may be, for example, an induction heater or gas-fired heater.

[0025] In addition, devices, (labeled here as 25 by way of example) for lubricating the roll gap are provided to reduce the friction between the work rolls. These devices are installed, specifically, only on the run-in side of the rolling stand. It is also advisable to use wear-resistant rolls, e.g., work rolls produced by the powder-metallurgical HIP process (high-isostatic pressing) or made of high-speed steels (HSS). In addition, it is advisable to install a strip surface inspection system 26 after the rolling mill and just before the coiler 20 and a camera to monitor the roughness of the strip and rolls. This monitoring system makes it possible to assess product quality online or during operation and, depending on the monitoring results, to determine the length, i.e., the duration, of the rolling program, which can thus be consistently extended with good quality results. In this regard, the surface inspection is preferably aimed at strip roughness and the rolling in of scale.

[0026] The loose scale formed during the rolling of carbon steels, which could be pressed into the surface of a subsequently rolled special steel strip, is removed by a rinsing system (labeled 27 by way of example) in the strip side guides. This system preferably operates in such a way that the wipers (not shown) of the rinsing system are intermittently swung out, and during this period of time the rinsing medium is applied to the surface of the roll and between the stands under high pressure by suitable high-pressure nozzles (not shown) in order to flush out scale or abrasive particles that may be present.

[0027] In addition, of course, the strip quality can also be improved by the use and controlled actuation of the section final control elements for shifting work rolls and adjusting roll bending.

[0028] It is proposed that the sequence of the entrance of a first thin slab, a group of first thin slabs, i.e., from the first process route I, of a second cast product or a group of second cast products into the rolling mill and the duration of the common rolling program, i.e., the period of time between the changing of the work rolls, be computer assisted on the basis of a process model. The use and duration of use of the grinding and polishing machines or of the rinsing systems for removing the scale of the carbon steels as well as of other systems designed to improve the strip surface are also controlled on the basis of this process model. A control unit of this type is designated schematically by reference number 28. In addition, the permissible jumps in the width of the cast products to be successively rolled can be taken into consideration in the rolling program according to “mixed rolling”. All together, a process model of this type allows optimization of the rolling quality as a function of the desired product quality.

[0029] The installation shown in FIG. 3 is the same as that shown in FIG. 1 with respect to its first process route I. In contrast to FIG. 1, the second cast products (2) integrated in process route I are cast with thicknesses up to 250 mm by a conventional casting machine 30 and, possibly after cooling and reheating in a furnace 31, rolled down in a primary rolling stand 32 to a thickness such that they can be integrated in the common rolling mill of process route I. The conventional sequence of continuous casting and primary rolling is shown only schematically here. The individual slabs are conveyed in the cold state or possibly while still hot to a slab furnace 21 near the first process route I, in which they are heated or homogenized to suitable rolling temperatures, and then inserted in the reheating furnace 12 with a ferry 22.

[0030]FIG. 4 is a schematic representation of a third embodiment of the installation used to carry out the common rolling program. In this embodiment, the integration unit is a ferry 220, which conveys both the first cast products (100) and the second cast products (200), which have been cut to length, into a common rolling mill 300 as each of these groups of products comes from the two reheating furnaces (120, 210) and thus from the process routes I and II.

[0031] All together, the process of the invention for processing special steels and carbon steels in a common rolling program with the use of surface-improving measures makes it possible to produce special steels more economically than state-of-the-art processes, because free capacities of existing installations that produce primarily carbon steels, i.e., such installations that are not operating at full capacity, can be utilized to supply other types of steel or cast products produced in another way. 

1. Process for producing steel strips and sheets, which comprises the following steps: casting a strand of carbon steel or special steel in thin slab format in a casting machine (4), passing the cast products (1) through a reheating furnace (12) to produce a uniform temperature, rolling the cast products (1) in a rolling mill (3), cooling the rolled product on a runout table (18), and coiling the rolled product on a coiler (20), characterized by the fact that, together with the first cast products (1), at least a second series of cast products (2), which is produced in an additional, second process route (II), is rolled within a common rolling program, such that, if the first cast products are cast from carbon steel, then the second cast products are cast from special steel, and if the first cast products are cast from special steel, then the second cast products are cast from carbon steel.
 2. Process in accordance with claim 1, characterized by the fact that the rolling mill is adapted to the given material to be processed during the rolling program.
 3. Process in accordance with claim 2, characterized by the fact that the rolling mill is adapted in such a way that it is prepared for the material to be processed next, while the preceding material is still being processed.
 4. Process in accordance with claim 2 or claim 3, characterized by the fact that the rolling mill is adapted in such a way that it is prepared for the next material during the rolling interruption between the rolling operation of two strips of different materials.
 5. Process in accordance with any of claims 1 to 4, characterized by the fact that the first cast product or groups of first cast products and the second cast products or groups of second cast products are alternately rolled within the common rolling program.
 6. Process in accordance with claim 5, characterized by the fact that the first cast products and the second cast products or a group of these products are rolled in more or less regular alternation within the common rolling program.
 7. Process in accordance with any of claims 2 to 6, characterized by the fact that, during and/or after the rolling operation on the products made of carbon steel, the rolling mill and the surface of the product that is to be processed or that has already been rolled are freed of scale, and that, during and/or after the rolling operation on the products made of special steel, the roll surface roughness that develops is reduced.
 8. Process in accordance with any of claims 1 to 7, characterized by the fact that the second cast products are also thin slabs, which are cast in a second process route (II) parallel to the first process route (I), cut into thin slabs, and then rolled with the first thin slabs in the common rolling mill in the order conforming to the common rolling program.
 9. Process in accordance with any of claims 1 to 8, characterized by the fact that, in the first process route (I), the first thin slabs are cast in the thin slab casting machine (4) and passed into the reheating furnace (12), and that, in the second process route (II), the second cast products (2) are cast in thicknesses up to 250 mm by a continuous casting machine (2), rolled down to thicknesses approximately equal to the thicknesses of the thin slab cast products (1), and then rolled with the first thin slabs in the common rolling mill (3) in the order conforming to the common rolling program.
 10. Process in accordance with claim 9, characterized by the fact that the primary rolled slabs, possibly after being conveyed to the installation in which the first process route is running, are reheated in a slab furnace (21) or, immediately after being primary rolled in a primary rolling stand (32), are conveyed by a ferry (22) into the reheating furnace (12) or directly into the common rolling mill (3).
 11. Process in accordance with any of claims 1 to 10, characterized by the fact that the order of entrance of a first cast product, a group of first cast products, a second cast product or primary rolled product, or a group of second cast products or primary rolled products into the rolling mill, the duration of the common rolling program, and the duration of use of the means (23, 24, 25, 26, 27) of adapting the rolling operation to the particular material to be processed are determined and adjusted.
 12. Process in accordance with any of claims 1 to 11, characterized by the fact that the operating time of the rolling program is determined as a function of the strip surface quality, which is ascertained by means of a strip surface inspection system (26).
 13. Installation for carrying out the process in accordance with any of claims 1 to 12, with a process line (I), which comprises a casting machine (4) for casting a strand in thin slab format from a carbon steel or a special steel, equipment (12) for heating and/or homogenizing the temperature of the cast products, a rolling mill (3) with at least one rolling stand (15 a) for producing the desired final dimensions, at least one runout table (18) for cooling the rolled product, and at least one coiler (20), characterized by at least one integration unit (22, 22 a, 220) for inserting at least second cast products (2) made of special steel, when the first cast products (1) are cast from carbon steel, or for inserting second cast products (2) made of carbon steel, when the first cast products (1) are cast from special steel, into a common process line, and by a rolling mill (3, 300) with a combined rolling program for the first cast products (1) and the second cast products (2).
 14. Installation in accordance with claim 13, characterized by the fact that it includes means (23, 24, 25, 26, 27) for preparing the rolling mill for the following cast product made of the other type of steel.
 15. Installation in accordance with claim 14, characterized by the fact that the means for preparing the rolling mill for products made of carbon steel comprise grinding and/or polishing equipment (23) for the work rolls of at least one rolling stand (15 a).
 16. Installation in accordance with claim 14 or claim 15, characterized by the fact that the means for preparing the rolling mill for products made of special steel comprise at least one rinsing system (27) for removing scale from the surface of the strip and the surfaces of the work rolls.
 17. Installation in accordance with claim 16, characterized by the fact that the rinsing system alternately uses wipers and nozzles for the application of rinsing medium under high pressure.
 18. Installation in accordance with any of claims 14 to 17, characterized by the fact that the means for preparing the rolling mill for products made of carbon steel or special steel include a device (25) for lubricating the roll gap, and that different lubricants are used for lubricating the roll gap for carbon steel and special steel.
 19. Installation in accordance with any of claims 14 to 18, characterized by the fact that the means for preparing the rolling mill for products made of carbon steel include one or more strip edge heaters (24), which are installed in front of the first rolling stand (15 a) or between the forward rolling stands of the rolling mill.
 20. Installation in accordance with any of claims 14 to 19, characterized by the fact that the work rolls of the rolling stand have high resistance to wear as a result of the use of rolls made by the powder-metallurgical HIP process or the use of rolls made of a high-speed steel.
 21. Installation in accordance with any of claims 14 to 20, characterized by the fact that it includes a control unit (28) for controlling the integration unit (22, 22 a, 220) between the two process routes (I, II) to adjust the order of entrance of the cast products (1, 2) into the common rolling mill (3) and for controlling the means (23, 24, 25, 26, 27) for preparing the rolling mill for the next product made of the other type of steel. 